NEW PERSPECTIVES ON THE GEOLOGY OF THE SAN JUAN ISLANDS FROM LIDAR AND SUBMARINE IMAGERY

The basic structural framework of the San Juan Islands has long been considered to consist of discreet, stacked thrust faults bringing together far distant terranes. Virtually no high angle faults are shown on any geologic maps of the region. However, recent lidar imagery reveals remarkable morphological details not visible by any other means. Faults and other geologic structures are clearly visible on lidar images, including many large fault scarps along the margins of the larger islands and many faults that criss-cross the islands. The straightness of the faults across topographic relief indicates that they are high angle faults.

Several long, linear, east-west-trending faults cut across Orcas Island. A continuous, straight, high-angle fault extends across the entire width of the island and another fault reaches from Doe Bay to West Sound. A fault scarp that makes the entire northeastern coastline of Orcas Island extends to depths of ~30m (~100) feet below sea level.

Three E-W faults cross the entire width of San Juan Island and several others extend part way across. Several parallel, high-angle, NW-SE–trending faults cut across southern Lopez Island. Their straightness across rugged topography indicates steep dips.

The morphology of the sea floor on sonar imagery shows many submerged faults as long linear scarps on the sea floor, some of which truncate geologic structures. Most of islands appear to be the tops of more extensive, submerged geologic structures bounded by large faults. Many bold, linear scarps on the sea floor occur between the islands, some having relief up to 300m (1000 ft). The San Juan-Lopez fault, the largest fault in the islands, extends for at least 65 km (40 mi) from Stuart Island to Lopez Island with a scarp up to 330m (1000 ft) high.

The age of these faults is not accurately known, but the topographic freshness of the scarps suggests that they are geologically young. Pleistocene continental ice sheets over a mile thick crossed the islands more than half a dozen times, but the scarps are not severely eroded,